Well, I have tryed to add -m32 to my CFLAG line. Still no luck. I put in a bug report, but they are saying to remove the -march from the ebuild. I tryed that, but it is still trying to use pentiumpro. How do I remove it? Here is the bug report for your viewing enjoyment.
http://bugs.gentoo.org/show_bug.cgi?id=37578

I would consider hitting up the mythtv user mailing list (here) and seeing if anyone else has run into the same problem or if they have any cvs patches for you to play with. The devs read that list too, so they'll probably pick up your question pretty quick.

I'm getting the same thing on my AMD64 install. That error message looks like it's gcc choking internally. I tried messing with optimization levels and nothing fixed it so I think we're dead in the water unless we can get one of the mythtv people to help.

this happens to me with tvtime on my pentium4.
It first happened when I first tried to emerge tvtime.

I then got it to emerge. Then when I did a "emerge world -U" it stopped because the tvtime upgrade wouldnt compile with similar errors

THE FIX
edit /etc/make.conf

and comment out CFLAGS and the one below it CXXFLAGS and emerge. It worked and I suspect it will for mythTV as well. Remember though that it won't be optimized (I don't notice in the operation of it tho!)

Remember to uncomment the above afterwards so other software gets optimized.

The problem with MythTV is that if you don't have the -march in the gcc line, then it puts in pentiumpro. You can't put in k8 otherwise everything fails. I was able to get it past one point only to fail another. And, yes, I am running .13-r1.